1. Field of the Invention
The present invention relates generally to contact electrode material used for a vacuum interrupter and a method of manufacturing the contact electrode material, and more particularly to a contact electrode material for a vacuum interrupter which can reduce the chopping current value inherent in contact material so that a small lagging current due to inductive loads can stably be interrupted without generating surge voltages.
2. Description of the Prior Art
Contact electrode material exerts serious influences upon circuit interruption performance in a vacuum interrupter. Generally, the contact electrode is required to consistently satisfy the following various requirements:
(1) Higher large-current interrupting capability; PA1 (2) Higher dielectric strength, PA1 (3) Excellent anti-welding characteristic; PA1 (4) Higher small lagging- or leading-current interrupting capability; PA1 (5) Higher electric conductivity, PA1 (6) Lower electrode contacting electric resistance; and PA1 (7) Excellent anti-erosion characteristic.
In the above requirements, the item (4), in particular, will be explained in more detail hereinbelow. In the case where an inductive load is connected to a circuit to be interrupted, current lags as compared with voltage in phase. The current lagging as compared with voltage is called lagging current. On the other hand, in the case where a capacitive load is connected to a circuit to be interrupted, current leads as compared with voltage in phase. The current leading as compared with voltage is called leading current.
In order to improve the above-mentioned lagging- or leading-current interrupting capability, in particular the lagging-current interrupting capability, it is indispensable to reduce the chopping current value inherently determined in contact electrode material provided for a vacuum interrupter. The above chopping current value will be described in detail.
When a small AC current is interrupted by an interrupter, a small-current arc is produced between two contact electrodes. When the small AC arc current drops near zero, there exists an arc current chopping phenomenon such that the current wave begins to vibrate and then is chopped (suddenly drops to zero) before the current reaches zero. An arc current I.sub.o at which vibration begins is called unstable current; an arc current I.sub.c at which current is chopped is called chopping current. In practical use, since this chopping current generates surge voltage, there exists a danger that electrical devices connected to the circuit interrupter may be damaged.
The reason why the arc current is chopped is explained as follows: When arc current reaches near zero, since the number of metal particles emitted from the cathode spots decreases below a particle density at which arc can be maintained, the arc current becomes unstable, resulting in current vibration and further current chopping. Since the chopping current generates harmful surge voltages, it is preferable to reduce the chopping current as small as possible.
The chopping current value decreases with increasing vapor pressure of the cathode material (low melting point material), because the higher the vapor pressure, the longer metal vapor necessary for maintaining an arc will be supplied. Further, the chopping current value decreases with decreasing thermal conductivity of cathode material, because if thermal conductivity is high, heat on the cathode surface is easily transmitted into the cathode electrode and therefore the cathode surface temperature drops abruptly, thus reducing the amount of metal vapor omitted from the cathode spot.
Therefore, in order to reduce the chopping current value, it is preferable to make the contact electrode of a material having a low thermal conductivity and high vapor pressure (low melting point). In contrast with this, however, in order to improve the large-current interrupting capability, it is preferable to make the contact electrode of a material having a high thermal conductivity and low vapor pressure (high melting point). As described above, since the high current interrupting capability is contrary to the low chopping current value, various efforts have been made to find out special alloys suitable for the contact electrode for a vacuum interrupter.
Description has been made of the mutually inconsistent relationship between large-current interrupting capability and small-current interrupting capability. However, there exists the other mutually inconsistent relationship between the requirements already stated above with respect to the contact electrode material for a vacuum interrupter.
For instance, U.S. Pat. No. 3,246,976 discloses a copper alloy for contact electrode, which includes bismuth (Bi) of 0.5 percent by weight (referred to as Cu-0.5Bi hereinafter). Further, U.S. Pat. No. 3,596,027 discloses another copper alloy for contact electrode, which includes a small amount of a high vapor pressure material such as tellurium (Te) and selenium (Se) (referred to as Cu-Te-Se, hereinafter). The Cu-0.5Bi or the Cu-Te-Se, including a high vapor pressure material, is excellent in large-current interrupting capability, anti-welding characteristic and electric conductivity; however, there exists a drawback such that the dielectric strength is low, in particular the dielectric strength is extremely reduced after large current has been interrupted. In addition, since the chopping current value is as high as 10 amperes, surge voltages are easily generated while current is interrupted, thus it being impossible to stably interrupt small lagging current. That is to say, there exists a problem in that electrical devices connected to a vacuum interrupter may often be damaged by the surge voltages.
On the other hand, in order to settle the above-mentioned problems resulting from the above Cu-0.5Bi or Cu-Te-Se, U.S. Pat. No. 3,811,939 discloses an alloy for contact electrodes, which substantially consists of copper of 20 percent by weight and tungsten of 80 percent by weight (referred to as 20Cu-80W hereinafter). Similarly, British Application Published Patent No. 2,024,257A discloses a copper alloy for contact electrodes, which includes a low vapor pressure material such as tungsten (W) skeleton (high melting point material) for use in high voltage. The 20 Cu-80 W or the copper-tungsten-skeleton alloy is high in dielectric strength; however, there exists a drawback such that it is difficult to stably interrupt a large fault current produced by an accident.